Whenever you carry out a reaction, you should stir it in order to obtain a homogeneous mixture. This becomes even more important when you heat solutions in order to prevent superheating. Usually the liquid on the bottom part is the one that is heated the most, because that's where the heat source usually is. Depending on the surface tension and other parameters, it is very difficult to form a gas. The agitation alleviates this problem a little bit and provides a smooth boiling.